RF shielding and eddy currents in NMR probes

Shielding of the NMR sample by a thin coating of metal between the sample and the rf coil arises in some geometries. The shielding may be used for rf heating of the sample tube or for a high infrared reflectivity coating in cryoprobe applications. An important application is for a shield that prevents noise from entering the rf probe circuit while allowing pulsed magnetic field gradients or field modulation to pass. We show by simple, approximate derivations that the criterion for shielding is not whether the coating exceeds the classical electromagnetic skin depth delta at the operating frequency (as is often stated), but whether the geometric mean between the thickness and an appropriate radius r exceeds delta. Thus, because r is typically much larger than delta, conducting layers substantially thinner than delta may still be good shields. Measurements are performed at high audio frequencies to confirm the calculations, using geometries relevant to rf saddle coils and to rf solenoids. Measurements of the slowing of the edges of a pulsed field gradient are also in accord with the calculations. (C) 2019 Elsevier Inc. All rights reserved.